Question

我正在阅读以下文章：http://java.sun.com/docs/hotspot/gc1.4.2/example.html并且无法理解以下几行：

Young generation size is too small

The young generation heap size in this first example is about 4 Mbytes with an overall heap size of about 32 Mbytes.

[GC [DefNew: 4032K->64K(4032K), 0.0429742 secs] 9350K->7748K(32704K), 0.0431096 secs]

[GC [DefNew: 4032K->64K(4032K), 0.0403446 secs] 11716K->10121K(32704K), 0.0404867 secs]

[GC [DefNew: 4032K->64K(4032K), 0.0443969 secs] 14089K->12562K(32704K), 0.0445251 secs]

This output seems reasonable from the point of view of the time spent in garbage collection but note that although the occupancy of the young generation is decreasing (e.g., in the first line from 4032 to 64k by 3968k) the occupancy of the entire heap is decreasing by considerably less (by 1602k from 9350k to 7748k). This indicates that only about 40% objects in the young generation were garbage and the rest survive the collection and are being promoted into the old generation.


Increasing the young generation size to increase the free space after the collection


The young generation heap size in this next run is increased to 8 Mbytes.


[GC [DefNew: 8128K->64K(8128K), 0.0453670 secs] 13000K->7427K(32704K), 0.0454906 secs]

[GC [DefNew: 8128K->64K(8128K), 0.0388632 secs] 15491K->9663K(32704K), 0.0390013 secs]

[GC [DefNew: 8128K->64K(8128K), 0.0388610 secs] 17727K->11829K(32704K), 0.0389919 secs]


With an 8 Mbyte size most of young generation is garbage at the time of the minor collection. In the first line the young generation heap decreases by 8064k from 8128k to 64k and the entire heap decreases by 5573k from 13000k to 7427k meaning about 68% of the young generation was garbage. As would be expected the size of the young generation does not change the amount of live objects that survive the minor collection (about 2.4M bytes in each case) but there are fewer minor collections and the cost of the collections in terms of the minor collection pause times are comparable (fractions of a second listed at the end of each line).

我的问题是，在这种情况下，增加YoungGen的大小如何帮助我们。应用程序在YoungGen中分配的对象总数将保持不变。

Answer 1

从YoungGen中提取一些东西是昂贵的，它会导致：

它活得更久，浪费记忆
未来的垃圾收集（每一个直到它已经死了）都要贵一些
YoungGen系列会忽略它，你必须等到耗尽更宝贵的资源，并迫使更昂贵的收藏。

通过增加YoungGen的大小，他们确保更多的对象被它收集，因此不会遇到上面的任何坏点。也就是说，对象快速死亡并且不会给任何人带来任何坏处，所以一切都更快？

此外，您正在阅读的文档有更现代的版本，尽管它似乎缺少您的示例： http://www.oracle.com/technetwork/java/javase/gc-tuning-6-140523.html

Answer 2

答案在于文章本身。

正如预期的那样，年轻一代的规模不会改变在次要集合中存活的活动对象的数量（约每种情况下都是2.4M字节）但是根据次要集合暂停时间，次要集合和集合的成本较少具有可比性（每行末尾列出的秒数）。

这是增加YoungGen的直接后果。

Answer 3

年轻收藏的成本与保留的对象数成正比。年轻集合越大，不再需要这些对象的可能性越大，这意味着您可以发现年轻集合的成本不会超过一定的大小。

对于年轻的小尺寸，它的比例相当，但对于较大的尺寸则不是那么多。收集频率与大小成比例下降。您可以达到每天收集不到一次的程度。 ;）

YMMV取决于您的应用程序的行为方式。

Java：增加YoungGen大小以提高GC性能

3 个答案: